Tone compensated volume control



N V- 6, 1945- D. E. SUNSTEIN 2,388,505

TONE COMPENSATED VOLUME CONTROL Filed April 29, 1944 kmusncy 6 7 '6 mo TATE/V517) DB Fmmuavcy INVENTOR.

Patented Nov. 6, 1945 TONE COMPENSATED VOLUME CONTROL David E. Sunstein, Elklns Park, Pa., assignor to Philco Corporation, Philadelphia, Pa.,

ration of Pennsylvania Application April 29, 1944, Serial No. 533,367-

a (Cl. 179-1) 4 Claims. The present invention relates to volume control circuits, and more particularly to a circuit in which the volume may be lowered without disturbing the tonal balance.

In home radio receivers the reproduction of sound is ordinarily considerably lower in level than the level of. the sound at the source. In order to reproduce sound at a lower level than the level at which the sound is generated, a tone compensation must be introduced in the reproducing system in order to maintain the same tonal balance existing in the original source of sound. Heretofore it has been shown by various authorities, including Fletcher, that the average human ear has a non-linear contour for a balanced relation of tones. This means that for the lower volume levels, the lower frequencies must be transmitted by a system having a rising characteristic below two thousand cycles,

Heretofore itv has been recognized that in order to permit a reasonably balanced tonal reproduction of sound at lower volume levels, it is necessary to introduce some compensation in the sound reproducing system. Quite frequently this has been introduced by combining on a single control shaft two resistors, one connected in the volume control circuit, and another connected in a control circuit arranged to vary the response characteristic of the-audio frequency transmission channel. In still other arrangements amulti-part circuit has been employed where the volume control is tapped at a certain tion circuit. While such circuit arrangement provides some base compensation'when the volume control arm is at or below the fixed tapped point on the resistor, this never has been regarded as entirely satisfactory. It, therefore, would be desirable to provide an improved circult arrangement which permits the volume level to be reduced without disturbing the tonal balance in any appreciable amount. It furthermore is desirable to provide such circuit in a relatively simple form so as to be economical to manufacture and be available to the general public.

In accordance with the present invention these and other objectives are obtained by providing a circuit in which a variable resistor connected vention to provide an improved tone compensated volume control which is simple and reliable in operation. I a

It is a further object of the present invention to provide an improved tone compensated volume control which approximates at the various volume levels the tonal balance required by the average human ear.

.Still another object of the present invention is to provide a tone compensated volume control circuit wherein a single variable resistor simul-' taneously functions to control volume. and to effect tone compensation in a predetermined desired manner.

Other and, further objects'of the present invention subsequently will become apparent by reference to the following description taken in connection with the accompanying drawing, wherein Figure 1 is a schematic representation of a tone compensated volume control circuit or network constructed in accordance with the present point so as to be connected to a base compensa-.

in series relation toan audio frequency trans- It, therefore, is an object of the present ininvention;

Figure 2 is a circuit diagram of the manner in which the basic circuit shown in Figure 1 may be incorporated in an audio frequency channel or amplifier;

Figure 3 is a graph in whichv the curves indicate the tonal balance'required by the average human ear at different volume levels; and

. Figure 4 is a graph showing various curves which illustrate the operation. of the circuit shown in Figure 1.

The frequency response of the human ear varies with the sound intensity so that any artificial'change in the volume of reproduced sound must be accompanied by 'a change in the frequency response of the sound reproducing system in order to maintain the same tonal balance existing in the original sound source. If it is assumed that the original sound intensity, as-for example in a radio broadcasting studio, is at a level of fifty decibels any reproduction of such sound at a level below fifty decibels would have a different characteristic as is best illustrated by the curves shown in Figure 3. The curves shown in Figure 3 are response curves obtained by taking the difference between a contour at a level of fifty decibels and the contour obtained at lower volume levels based upon the response curves for the average human ear as .given by Fletcher. Thus the curve A for reproduction at fifty decibels appears as a straight line where the abscissa are in frequencies covering a range from thirty to ten thousand cycles per second.

Curve B shows the curve obtained through the sound level of forty decibels. From the forty decibel volume level curve, it will be seen that the sound intensity required for a tonal balance comparable to the reproduction at fifty decibels requires a higher relative gain in the region below two thousand cycles. At lower volume levels, such as at thirty, twenty, ten or zero decibels as shown by curves C, D, E and F, there is still a greater difference required between the gain for the tones of frequencies below two thousand cycles and those above two thousand cycles. In order to provide a balanced reproduction at lower volume levels, it is necessary to provide an audio frequency current transmission network or channel which will introduce a compensation into the reproducing system so that the response curves obtain approach those shown in Figure 3.

A circuit which approaches the desired results indicated in the curves of Figure 3 is shown in Figure 1 wherein a symbol of the generator ii represents a source of audio frequency currents which would produce in a loud speaker an intensity of fifty decibels. The source ii is connected to a transmission network which includes a series connected adjustable resistor l2, and a shunt circuit comprising a fixed resistor l3 and a capacitor it arranged across the output terminals i and it. It is assumed that the voltage appearing across the terminals i5 and it when the resistor l2 has been adjusted to zero value is sufficient to produce in a loud speaker energized from the output terminals l5 and iii a volume having an intensity of fifty decibels. The ratio of the output voltage appearing across the terminals l5 and it to the input voltage supplied by the generator i i is given by the following equation:

Therefore, the db attenuation is:

where R is resistor I3 R1 is resistor l2 C is capacitor l4 db attenuation [log (1 (wcRiwc R 2 log (1+1D C (R+Rl) ,If the resistor l3 and the capacitor M are so chosen as to have a time constant of the response produced by the reproducingdevice connected across the'output terminals l5 and [8 corresponds to that shown in Figure 4. When the resistor l2 has been adjusted to zero value the reproduction is at a volume level of fifty decibels indicated by the straight line G. When the resistor I2 is adjusted so asto have a value equal to the resistor I3, the response follows the curve H and the curve I shows the response obtained when the resistor l2 has a value three times as great as-the resistor IS. The curves J, L. M, N,.

' appear substantially parallel in Figure 4, whereas in Figure 3, the curves are not parallel. This difference between the lower intensity curves in Figure 4 produces an eflect which gives an excessive amount of low frequency reproduction in the vicinity of one hundred cycles at about a. thirty to forty decibels volume level.

In order to further compensate the reproduction of sound at the lower volume levels in the region of one hundred cycles a resistor approximately five times the value of the resistor 13 may be connected across the output terminals i5 and iii. Another manner ofaccomplishing a similar result or effect is to double the value of the capacitor i i and to connect a capacitor having the same value in series with the resistor l2. This then produces a compensation which very closelyapproximates the various .curves shown in the graph of Figure 3 at various volume levels with the exception of the very low levels in the vicinity of curves E and F where the reproduction will be slightly deficient in intensities of low frequencies.

For a practical application of the circuit of Figure 1 as further modified to produce the ultimate desired response characteristics, reference may be had to Figure 2 which shows audio frequency input terminals H and i8 which may be obtained from a detector or a phonograph pickup. The terminal ii is capaoitively coupled by a capacitor iii to the control grid of a vacuum tube 2|. The grid-to-cathode circuit of the vacuum tub 2i includes a resistor 22, and the cathode is connected to ground through a resistor 23. The anode of the vacuum tube 2! is supplied by voltage from a suitable source and is by-passed to ground by a capacitor 24. The audio frequency component appearing across the cathode resistor 23 comprises a low impedance source of quency response produced by the transmission network and are included in the circuit to keepmegohm.

audio frequency currents which are to be supplied to the audio frequency transmission network embodying the present invention. This network includes a series capacitor'2l5 connetced in series with an adjustable resistor 28 which is connected to the control grid ofia succeeding audio frequency amplifier tube 21. A shunt circuit comprising a resistor 28 and a capacitor 2! is effectively connected across the output circuit of a network which is connected to the input circuit of the amplifier which includes the vacuum tube 21. The capacitors 25 and 29 may be of equal value each having a value two times as great a the capacitor it of Figure 1. The fixed resistor 28- corresponds to the resistor ll of Figure 1, and the adjustable resistor 28 corresponds to the adjustable resistor l2 of Figure 1. A shunt resistor 3| is connected between the grounded conductor of the network, and the juncture between capacitor 25 and adjustable resistor 28.

The resistor 3| has relatively high resistance values compared to the resistor l3. 7 Another resistor 32 is connected across the output of the network and this resistor has a value even greater than the resistor 3|. The resistors 34 and 32mm thus be made to have a negligible eflect on the frothe maximum direct current resistance in the grid circuit of the first stage of the audio frequency amplifier, which may include the amplifier tube 21, to a value in the vicinity of one v Further application of the transmission network to a practical amplifier will be readily appreciated by an example of typical values of the .so as to have a long life.

resistors and capacitors employed. 'It may be seen, for example, that the input resistor 23 which is a cathode load resistor for a vacuum tube has a value or about ten thousand ohms. The capacitors 25 and 29 each have a value of .32 microfarad's which gives an eflective series capacity of .16 microtarads, so that, together with the resistor ZB having a value of one thousand ohms, the time constant of the circuit is expressed by the equation ably is kept at a relatively low value of the order of one thousand ohms so that theresistor 26 will be correspondingly small in resistance value For the practical application of the circuit as illustrated by the example given in connection with Figure 2, the over-all gain of the system should be such so that with the value of the resistor 26 adjusted to zero, the average loudness level of reproduced sound at the listener would be'fiity decibels. In

a radio receiver embodying such compensation circuit the automatic volume control or automatic gain control applied to the radio frequency stages of the receiver should be substantially flat so that no gain control adjustmentneed be 'made when tuning from one station to another.

Where the circuit is to be used in a manner as illustrated by Figure 2 in connection with a phonograph reproducing system, it may be desirable to produce a high frequency cutoflabove five thousand cycles per-second to reduce noise and high frequency distortion. Such cutofl may be embodied in the amplifier, which includes the vacuum tube 21. should have a rising characteristic from approximately fifteen hundred cycles to tour or live thousand cycles in order tomaintain approximate tonal balance. x

From the above description it will be seen that there has been provided an audio frequency transmission circuit or network which is relatively simple, and in which a single variable resistor serves to control the volume level and simultaneously operates to introduce a compensat- I-Iowever, in such event this ing eil'ect'into the reproduced sound so as to maintain subs tially the same tonal balance existing in the original sound source. Such circuit arrangement, therefore, eliminates the neoessity of supplying a tone control device to compensate for changes in volume level in a phonograph. or radio receiver. Th base compensation is such that kettle drum, bass viols, and other orchestral instruments generating low frequency sounds will appear in the same general prominence regardless of the volume control setting.

While for the purpose oi illustrating and describing the present invention; certain specific embodiments have been disclosed, it is to be understood that the invention is not to be limited- .thereby since obviously such variations in circuit arrangements and in theinstrumentalities employed may be made as will be commensurate with the spirit and scope of the invention as defined in the appended claims.

The present invention is hereby claimed as follows:

1. In an audio frequency system, a source of audio frequency signals, a volume control circuit constructedand arranged to maintain an apparent tonal balance throughout the volume range, said volume control circuit having a pair of input terminals and a pair of output terminals;

means for coupling said source to said input terminals, avariable resistor connected between one of said input terminals and one of said output terminals, and a fixed resistance and a fixed capacitance connected in series across said output terminals, the time constant of said series-connected elements being 0! the order of second, said variable resistor being adjustable be- 40 t'ween a minimum value or approximately zero abalance. 2. An audio frequency system. as claimed in ohms and a maximum value 01' the order of hundreds of-times the value of said fixed resistor,

whereby variations in volume level maybe e1- iected without substantially disturbing the tonal claim 1. characterized in that said fixed resistor ha'sa value oi the order of one thousand ohms.

3. An audio. irequency system as claimed in claim 1, wherein said variable resistor is variable between the limits or approximately zero and one million ohms.

4. An audio frequency system as claimed in claim 1, characterized in that the said source comprises the load resistor of a cathode-loaded amplifier/stage. nsvmnstms'rnm. 

